This invention relates to a laser beam conditioner which is to be used in conjunction with a laser beam homogenizer for the purpose of reducing the interference contrast of the homogenized laser beam and so providing a more uniform intensity profile to the homogenized laser beam More particularly, this invention relates to a laser beam conditioner using partially reflective mirrors.
Excimer lasers have a wide range of applications involving etching and shaping of a variety of materials by annealing or ablation. For example, excimer lasers can be used in semiconductor annealing, microphotolithography and photodeposition. Lasers provide an intense light source which can be precision controlled to etch and shape of materials.
However, the effectiveness of an excimer laser in annealing and ablation requires a uniform laser beam profile to produce a smooth and even energy distribution across the material.
Two problems exist in providing a uniform intensity profile to a laser beam. First, the beam emitted by an excimer laser has a Gaussian intensity profile, peaking at the center of the beam and gradually decreasing toward the circumference of the beam. Second, the beam emitted by an excimer laser is coherent. A coherent beam has a finite and measurable coherence length. Any overlapping of a coherent beam with itself creates interference standing waves in the beam profile. These interference standing waves create an intensity modulation or an interference fringe pattern in the beam's intensity profile making the profile non-uniform. An ideal laser beam for annealing and ablation has a uniform intensity profile without any interference modulation.
There are numerous ways to provide a uniform intensity profile to a laser beam emitted by an excimer laser. Most beam homogenizers, particularly the economical and easy to manufacture ones, require overlapping the laser beam. In a typical homogenizer, the laser beam is divided into multiple wavelets, which are then redirected to different positions and overlapped back to produce a beam with a more uniform, less Gaussian intensity profile. This process is referred to as beam folding. Because beam folding produces interference standing waves and an interference fringe pattern, it is desirable to provide a beam conditioner which precedes the homogenizer in the optical path of the laser beam and which reduces the interference contrast caused by the homogenizer.
One means to reduce the interference contrast of the beam profile is found in U.S. Pat. No. 5,153,773 (hereinafter referred to as the "Muraki et al." patent). The beam exposure apparatus of Muraki et al. consists of a polarization beam splitter and two mirrors. A coherent light beam from an excimer laser is inputted to the polarization beam splitter. The S-polarized portion of the light beam is reflected by the beam splitter while the P-polarized portion of the light beam is transmitted through the beam splitter. The transmitted P-polarized beam is reflected by the two mirrors back to the polarization beam splitter. The P-polarized beam is again transmitted through the beam splitter to overlap the S-polarized beam.
The optical path difference (OPD) in Muraki et al. between the initial reflected beam and the initial transmitted beam which circulates once between the mirrors and then emerges from the beam splitter is greater than the coherence length of the laser beam. Accordingly, the intensity profiles of the two beams will sum incoherently to reduce the coherence of the resulting beam while improving the homogeneity of the beam.
Muraki et al. also provides an alternate means of beam homogenizing by using a single polarizing beam splitter prism rather than the three element apparatus of the polarizing beam splitter and the two mirrors.
However, a polarizing beam splitter and, more especially, a polarizing beam splitter prism with the necessary high level of optical efficiency for Muraki et al. to work is expensive and difficult to manufacture. Also, additional optical elements such as mirrors are need to make the output homogenized light beam parallel, if not coaxial, with the input beam.
Another arrangement to reduce the coherence of a laser beam is U.S. Pat. No. 5,343,489 (hereinafter referred to as the "Wangler" patent) and U.S. Pat. No. 5,224,200 (hereinafter referred to as the "Rausmussen et al." patent). Wangler and Rasmussen et al. both use two parallel mirrors as a coherence reducer. One mirror is fully reflecting and the other mirror is partially reflecting with the reflectance varying along the length of that mirror either in precise steps or varying in a continuously changing geometric curve. An entering laser beam will reflect between the two mirrors with portions transmitted through the partially reflecting mirror to form a reduced coherent beam.
The inventions of Wangler and Rasmussen et al. are not practical for excimer laser beam conditioning because both inventions expand the beam to a size many times its initial cross section. Typical excimer lasers emit beams with cross sections of several centimeters, and at fluences which make substantial beam condensation impractical due to the damage threshold of most optical coatings. The condenser lens system in Wangler and in Rasmussen et al. needed to focus these expanded beams would become prohibitively large in a device used for excimer laser beam conditioning. Also, the constantly varying reflectance of the partially reflective mirrors in Wangler and in Rasmussen et al. will be expensive and difficult to fabricate.
It is an object of this invention to provide an optically simple laser beam conditioner to be used in conjunction with a laser beam homogenizer which can reduce the interference contrast caused by beam folding while providing a substantially parallel and coaxial beam.